Silicon carbide (SiC) is a semiconductor that has excellent material property values in which the band gap of silicon carbide is approximately three times as large as that of silicon (Si), the saturation drift speed of silicon carbide is approximately twice as high as that of silicon, and the dielectric breakdown field strength of silicon carbide is approximately ten times as strong as that of silicon. Accordingly, silicon carbide has been developed as a material of a semiconductor device for electric power, and a Schottky diode formed by using silicon carbide is put on the market at present.
The Schottky diode is composed of an SiC monocrystal substrate that has been obtained by slicing an SiC bulk monocrystal into wafers where the SiC bulk monocrystal is grown by the sublimation method, an epitaxial layer that has been obtained by growing an SiC monocrystal film on the surface of the SiC monocrystal substrate with the chemical vapor deposition (CVD) method, a Schottky electrode that has been formed on the surface of the epitaxial layer by the sputtering method, vacuum deposition method, or the like, and an ohmic electrode that has been formed on the rear surface of the SiC monocrystal substrate. The Schottky electrode is made of a material such as nickel or titanium (see the patent document 1).
The power loss must be reduced for a Schottky junction type semiconductor device that is used for electric power such as a Schottky diode. The power loss of the Schottky diode depends on the Schottky barrier height (SBH) at the junction interface of the Schottky electrode and the SiC epitaxial layer. The power loss of the Schottky diode is based on the sum of the power loss at a forward direction flow of an electric current and the power loss caused by a leak current at an action of a reverse voltage.
For instance, the power loss density of the Schottky diode is described as ½ (VfJf+VrJr) at a 50% duty cycle (see the non-patent document 1). Here, Vr is a reverse voltage, Jf is a forward current, Vf is a forward voltage, and Jr is a reverse current. The evaluation of the Schottky diode is represented with Vr and Jf. On the other hand, Vf and Jr depend on the Schottky barrier height. In the case in which the power loss of a 4H—SiC Schottky diode is calculated where Jf is 100 Acm−2 and Vr is 4 kV for example, the power loss of the 4H—SiC Schottky diode becomes minimum when the Schottky barrier height is in the range of 1.18 to 1.3 eV at a temperature of the range of 25 to 200° C.
A Schottky diode with the reverse withstanding voltage of the range of 0.6 to 5.0 kV is used in many cases. For such a reverse withstanding voltage, the power loss becomes a minimum in the case in which the Schottky barrier height is in the range of 0.9 to 1.3 eV. However, in the case in which the Schottky electrode is made of nickel or titanium, the Schottky barrier height is approximately 1.6 eV for nickel and 0.8 eV for titanium. Consequently, the power loss of the Schottky diode cannot be a minimum.
It is proposed that a Schottky electrode made of titanium is formed on the SiC layer and a heat treatment is performed at the specified temperature in order to control the Schottky barrier height. However, in the case in which a Schottky electrode made of titanium is formed and a heat treatment is performed, the value of an ideal factor (n-factor), which is a parameter indicating the performance of the Schottky diode, is increased and extremely deviated from the ideal value 1 as shown in FIG. 2.
In general, in the case in which an electric current, which passes through the Schottky barrier interface, passes only over the barrier, that is the case of only the thermal diffusion current transportation, a current increases in an exponential function manner in comparison with a voltage. The value of an electric current is expressed by exp(eV/kT)−1; here, e is a prime electric charge, V is a voltage, k is the Boltzmann's constant, and T is a temperature. However, in the case in which an electric current passes not only over the barrier but also through the barrier by tunneling, an electric current flows even if a voltage is low, and the value of an electric current is deviated from the above expression. Consequently, the voltage V is replaced with V/n to visually correspond to the expression, thus expressing the value of an electric current as exp(eV/nkT)−1. The item n is an ideal factor and the value of n is 1 in the ideal case of only the thermal diffusion transportation current. However, the value of the n-factor is larger than 1 in the actual case in which other electric currents flow by many kinds of factors.
As described above, in the case in which a Schottky electrode made of titanium is formed and a heat treatment is executed at the specified temperature in order to control the Schottky barrier height, the value of the n-factor becomes greatly larger than 1, thus deteriorating the performance of the Schottky diode and increasing a leak current at an action of a reverse voltage.
Patent Document 1: Japanese Patent Laid-Open Publication No. 2000-188406
Non-patent document 1: IEEE Trans. Electron Devices, March 1993, Vol. 40, third issue, p. 645-655
Non-patent document 2: IEEE Trans. Electron Devices, April 2002, Vol. 49, fourth issue, p. 665-672